Sensitive discrete and imaging detectors for X-ray and medium to long wavelength infrared or sub-millimeter radiation have been in great demand, especially for astronomy studies. The most sensitive devices are calorimetric, based on the measurement, with a bolometer, of a temperature rise caused by the absorption of said radiation, and require the minimization of the thermal mass of the detector in order to maximize the temperature rise. For terrestrial applications there is a need for thermal infrared detectors simpler and less expensive than the ones so far available. Thermal detection of X-ray photons with energies of the order of 1 KeV or higher has progressed to the point that one can measure the temperature rise generated by the absorption of a single X-ray photon, and the measuring devices are known as “quantum calorimeters”. Thermal detection of medium or long wavelength infrared or sub-millimeter radiation is based on the same principles, but the energy of an infrared photon is several orders of magnitude lower than that of an X-ray photon, so a thermal infrared detector is not, strictly speaking, a quantum detector, and typically requires the absorption of a relatively large number of infrared or sub-millimeter photons. A thermal detector of infrared or sub-millimeter radiation comprises two elements: (a) an absorber of the radiation, usually a relatively thin (from less than one to a few micrometers) dielectric film coated with a thinner metal film, and (b) an associated temperature probe. The temperature rise measured by the probe is inversely proportional to the thermal mass of the detector. For a given weight mass, the thermal mass can be minimized by operating at liquid helium temperatures, where the heat capacity of the detector is approximately proportional to T3, where T is the absolute temperature in kelvins. The most sensitive detectors are, therefore, those that work at temperatures lower than 1K. On the other hand, many applications of infrared detection and/or imaging involve infrared intensities high enough that, although they still require absorbers of low thermal mass, they don't require cryogenic cooling of the detector.
Some detectors for thermal infrared radiation of wavelengths between about 8 and 14 micrometers, and suitable for infrared imaging, have used as the infrared absorber an approximately 2 nanometer thick permalloy (a nickel-iron alloy) film deposited on a thin (about 250 nanometers) silicon nitride (Si3N4) film (see for example U.S. Pat. No. Re. 36,706, a reissue of the 1994 U.S. Pat. No. 5,286,976). Thus, for a typical pixel area of about 7×10−6 cm2 the absorber mass per pixel (including the Si3N4 film) was not much greater than 10−9 grams. As known to workers with at least average skill in the art, such values are not substantially greater than minimally needed for the capture of a desired fraction of the intensity of said infrared radiation incident on the detector. Thus, the mass of the photoluminescent temperature probe of this invention, for said thermal infrared detectors and imagers, is no greater than 10−10 grams per pixel, and may be about an order of magnitude smaller.
An important recent advance in detectors for long wavelength infrared and/or sub-millimeter radiation was the substantial reduction of the thermal mass of the absorber through the use of an essentially planar metalized micromesh geometry reminiscent of a spider-web, as described by Mauskopf et al. in the journal Applied Optics 36, pages 765-771 (1997). This reduces the mass of the absorber to a fraction of the mass of a continuous absorbing film (this fraction has been called “the fill factor”, and this term shall be used in this disclosure). But there was no comparable advance in the reduction of the thermal mass of the temperature probe. In fact, the micromesh absorber now leaves the temperature probe as the largest component of the detector thermal mass in the art prior to this invention. And if the probe is electrical, as are the temperature probes in existing radiation detectors, it is also the main source of noise in the detector system, due to Johnson noise and/or Joule heating.